SAN DIEGO — Californians are bundling up with sweaters and gloves and stocking up on firewood as they brace for several nights of very unseasonable freezing temperatures.

The National Weather Service is forecasting morning frost on San Diego beaches. Big Sur, on the central coast, prepared for daytime highs almost 20 degrees below Boston’s. Even the snowbird haven of Palm Springs faced the possibility of freezing temperatures at night.

In addition, San Diego zookeepers turned up the heat for chimpanzees, tourists covered their hands on Hollywood walking tours, and some farmers broke out wind machines and took other steps to protect crops from freezing.

Freeze warnings were in effect in San Diego County valleys and deserts Saturday morning with lows in the 20s and 30s, the weather service said.

In Sonoma County, homeless shelters started handing out extra warm clothes on Friday to protect people from freezing overnight temperatures.

Morning temps fell into the 20s and 30s in many areas, and much lower in the mountains. A low of 12 degrees was recorded in the Big Bear mountain resort east of Los Angeles.

Some customers drove more than an hour to buy firewood.

“It’s crazy busy here,” said Renea Teasdale, office manager at The Woodshed in Orange, south of Los Angeles.

Still, it was business as usual as much of the state contended with temperatures in the high 40s and low 50s.

“It’s still sunny Southern California, and I’m going to work on my legs all year long,” said Linda Zweig, a spokeswoman for the Del Mar Fairgrounds, which is hosting a 5-kilometer run north of San Diego on Sunday. The lifelong San Diego-area resident is prone to wearing two sweatshirts when the temperature drops but refuses to give up on shorts.

In the San Joaquin Valley, the heart of California’s citrus production, growers prepared for another round of freezing temperatures late Friday after seeing little crop damage Thursday night.

They run wind machines and water to protect their fruit, which can raise the temperature in a grove by up to 4 degrees, said Paul Story, director of grower service at California Citrus Mutual. Existing moisture, sporadic rain and cloud cover can also help keep in heat.

Snow shut a 40-mile stretch of a major highway north of Los Angeles on Thursday afternoon, forcing hundreds of truckers to spend the cold night in their rigs and severing a key link between the Central Valley and Los Angeles.